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A New Simpler Blood Test To Screen For Colorectal Cancer May Help Save More Lives

Colorectal cancer (CRC) remains one of the most common and deadly forms of cancer in the United States. That sobering reality is in no small part because we don’t do enough screening to find the cancer at its earliest stages when it is most treatable or find and remove a colon polyp before it turns into cancer.

Despite all the awareness and all the attention, we don’t get enough folks screened. For almost all of us who are 45 years of age and older and are at average risk of the disease (meaning we don’t have a strong family history, other predisposing illness such as ulcerative colitis, or a known familial genetic predisposition) we could do a lot more to improve our chances of a better outcome if diagnosed with colorectal cancer. And that better outcome starts with being up to date on our CRC screening programs.

A new blood test for the early detection of CRC just released by Guardant Health, Inc. called “Guardant Shield“ may well be a significant step forward in our journey to get better control of our health when it comes to reducing the burden of CRC.

Part of the problem is that the tests we do have available to screen for CRC aren’t the most consumer/patient friendly: some require getting the fecal immunochemical test (FIT test) every year by fishing around the toilet bowl to get a stool sample, then sending it to the lab for analysis; another approach is to send your bowel movement in a bag every three years packed in a box to another lab for analysis of genetic abnormalities and the presence/absence of blood in the stool; and the third—and generally most widely used form of colon cancer screening–is to get a colonoscopy every 10 years, so long as no polyps or other growths are found which would then require more frequent follow-up.

Most people don’t like any of these options. None of them are easy or fun. And colonoscopy can be very inconvenient and expensive: you must take a day off work to get the test; you have to take an uncomfortable prep the night before, and you have to find someone to also take time from their work schedule to escort you to and from the colonoscopy suite since you will likely have sedation or anesthesia and consequently can’t drive yourself home.

Colonoscopy is also expensive, although many people think it is “free” because their insurance covers the procedure. I assure you the patient may not have to pay for the test, but nothing is free or inexpensive. My own colonoscopies—aside from the inconvenience—were well over $5000 and higher. And getting them scheduled at an acceptable time for me, my caregiver and the colonoscopist has not been a simple process.

We need alternatives for CRC screening that work, effective alternatives that could reduce the burden of colorectal cancer screening and hopefully get more benefit to more people, maybe even at lower total cost.

For the past several years, our scientific eyes have turned to another option: blood tests. Blood tests would be a more effective way of reaching more people to find CRC early, but they haven’t exactly been great performers. Although one is approved by the Food and Drug Administration it isn’t exactly an “early detection test” and hasn’t gotten great traction in the medical community.

On the other hand, there has been considerable excitement in the research community about harnessing the power of circulating tumor DNA to characterize cancers and their genetic abnormalities by studying the cancers themselves as well as blood samples to find opportunities to offer very specific treatments for cancer. This same technology is also being used to monitor the status of a cancer once removed and/or treated to determine whether there is any sign of cancer left in the body or if the cancer is returning, and if the cancer is returning whether a new treatment would be a better option based on the characteristics of that cancer.

It is not a huge leap of faith to imagine that this same DNA technology and its modifications could be considered a next step in finding cancers anywhere in the body before they became otherwise visible or problematic. And that’s what this new blood test is all about.

As time goes on, our science and our technology get better. We develop tools that can dig deeper into the blood to pull out smaller and smaller fragments of material which could signal that a cancer is present. It’s not easy to do, but just like taking radio waves from the air and passing them to a radio, which was once thought impossible and is now an everyday event, so we are getting to the point where we can pull these small protein fragments from the blood and see whether they indicate a cancer is present.

The new blood test from Guardant used to detect colon cancer is one of those advances: using the Guardant Shield blood test to look for signals which detect colon cancer at its earliest stages is an important, evolutionary strategy and technology. And it does so very well, especially when compared to other tests:

In a recently completed analysis undertaken by Guardant with information from patients who had undergone testing and were known to have either various stages of colon cancer or no cancer, the blood test found an existing cancer 91% of the time when it was present—particularly in earlier stage cancers–and a specificity of 92%, which means 92 times out a hundred when the test signaled a cancer was not present, a cancer was in fact not present.

That is not a perfect score, but it is pretty darn good, especially when compared to other tests that detect colorectal cancer early.

For example, the numbers for colonoscopy show a sensitivity of 95% and a specificity of 86%; for a stool FIT test—the one where you scoop a sample of stool out of the toilet once a year and send it to a lab—the numbers are 74% sensitive and 96% specific.

What is clear from this early report is that the new blood test from Guardant is reasonably effective when compared to other options. And that’s a good thing, however not the entire story.

There is another study currently underway where people at average risk of colorectal cancer who are undergoing screening are having the blood test done at the same time. The results of this study should be available later this year and given the design of the study and the large number of folks involved it should give us an even more accurate, real-life idea of how the new Guardant Shield test performs in more typical clinical scenarios compared to “look back” studies where the colonoscopy outcome is already known.

There is another very important part of this discussion that must not be ignored:

The Shield test is currently being recommended as an option ONLY for those folks who won’t get another standard accepted test, such as FIT or colonoscopy or FIT with genetic analysis. That would be the 33% of people who haven’t been screened as recommended and refused to be screened with a recommended test for whatever reason. If we could “convert” some of those folks to get the blood test then perhaps we will be able to improve our CRC screening rates even more than we have to date, and that may translate into saving more lives.

It is also important to note that if the Guardant Shield blood test is positive, then a follow up colonoscopy is mandatory: the blood test is a signal that something may be wrong; it takes a colonoscopy and careful examination of the entire bowl to find out exactly what is going on and what must be done to address any abnormality that is found. And, like the other screening tests, the Guardant test must be repeated regularly to be effective. Currently, the planned recommendation is to repeat the test every year.

We face a major problem today with CRC screening: we were not getting enough people screened even before the pandemic, and the pandemic certainly hasn’t helped. We don’t have enough slots available to accommodate everyone who wants a colonoscopy when they want it. The availability of colonoscopy varies considerably across the country based on geography and access in cities and whether enough specialists are available to perform it. And there remain those who simply want nothing to do with it.

The Guardant Shield test at this point in time is designed to help address some of those issues, including making a test available in locations where the opportunity to have a CRC screening test is limited for whatever factor. And with more data to come, there is the hope that this test will move forward to get full approval from the Food and Drug Administration and subsequent payment from Medicare, Medicare Advantage plans, and commercial insurers along with inclusion in major guidelines from organizations like the United States Preventive Services Task Force.

Over time, the technology will improve even further. And it is not outside the range of possibility that in the future we will have blood-based tools to find more cancers such as pancreatic and ovarian cancer earlier and more accurately than is the case today. I have no doubt that will happen—what I don’t know is how quickly. But this is a start, a very important start. (I should mention that Guardant Health has already announced their efforts in developing a screening test for lung cancer>)

Meanwhile, this is one more step in reducing the harms and burden of CRC, a very treatable disease—and acknowledging Guardant Shield as a piece of progress to save lives that CAN be saved through the benefits of early detection of colorectal cancer.


Dr. Lichtenfeld is a paid advisor for Guardant Health Inc. Although generally compensated for his time, he received no request or payment from Guardant to write this blog. He has been compensated for participating in professional webinars discussing Guardant Shield’s commercial release. The opinions in this blog are his and his alone, informed by publicly available information provided by Guardant press releases and websites.